The present invention relates to a new process for the production of N-substituted aminomethylphosphonic acid derivatives represented by the formula (I) ##STR3## wherein R.sub.1 is an alkyl having 1-18 carbon atoms or a cycloalkyl having 3-18 carbon atoms and R.sub.2 is hydrogen, methyl, or ethyl. Particularly this process provides methylaminomethylphosphonic acid dimethylesters and diethylesters, and the corresponding acid in high yield.
The known methylaminomethylphosphonic acid of the formula (II) ##STR4## has interesting applications for fire proofing and as a herbicide; see, e.g., U.S.S.R. No. SU 1,074,886 (Chemical Abstracts, 100, 19374C (1984)) and Ger. Offen. No. 2,848,869 (Chemical Abstracts, 93, 39529Z (1980). Although several processes for the production of these N-substituted-aminomethylphosphonic acids have been patented and published, all of them are complicated with several disadvantages.
The process of U.S. Pat. No. 2,328,358 involves reaction of N-methyl-N-hydroxymethylstearamide with PCl.sub.3, followed by the treatment with dilute hydrochloric acid to afford N-methylstearamidomethylphosphonic acid of the formula (III). ##STR5## This is then hydrolyzed to methylaminomethylphosphonic acid. This process requires excessively lengthy reaction times. The use of stearamide makes this process bulky and expensive.
In U.S. Pat. No. 3,907,652, the N-tertiaryaminomethylphosphonic acid of the formula (IV) ##STR6## is prepared from methylamine, formaldehyde, and phosphorous acid (H.sub.3 PO.sub.3) [Journal of Organic Chemistry, 31, 1603 (1966)] and followed by oxidative electrolysis to methylaminomethylphosphonic acid with removal of a phosphonomethyl group. This process requires special equipment for the latter reaction, which is done in strong acid.
The process of U.S. Pat. No. 4,351,779 involves the condensation of trimethylhexahydro-s-triazine with excess secondary dialkyl phosphite by heating at 20.degree.-150.degree. C., followed by hydrolysis or pyrolysis at high temperature (230.degree.-240.degree. C.). The source of phosphorus in this process, the secondary dialkylphosphite, is too expensive for commercial production.
In the process of U.S. Pat. No. 4,160,779, methylaminomethylphosphonic acid is prepared from the reaction of bis(chloromethyl)phosphonic acid in aqueous ammonia at 150.degree. C. and 80 bars pressure for seven hours. This requires a high pressure autoclave, which is an expensive piece of equipment. Also, purification of the crude product requires an acid ion exchanger. These requirements make this process inadequate for large scale production.
Two other processes involve the reaction of amide with phosphorus trichloride. The disclosure by Tyka and Hagele [Synthesis, 218-19 (1984)] is a laboratory procedure for the synthesis of N-alkylaminomethylphosphonic acid from N-alkyl-N-hydroxymethylformamides with phosphorus trichloride. This procedure is disadvantageous for the preparation and isolation of N-alkyl-N-hydroxymethylformamide in view of its low yield, making it adequate for industrial use. Freeman claims in U.S. Pat. No. 4,830,788 that this problem is solved by making the amide having the formula V ##STR7## or its low molecular weight carboxylic ester as formula VI. ##STR8## The amide for the production of methylaminomethylphosphonic acid is prepared in situ from N-methylacetamide, acetic acid, acetic anhydride, and paraformaldehyde by heating to 116.degree. C. Then, the reaction mixture is cooled to 25.degree. C. before adding phosphorus trichloride dropwise. Near the end of the PCl.sub.3 addition, temperature is maintained at 59.degree.-70.degree. C. for forty five minutes and then gradually increased to 130.degree. C. for 3 hours. This process requires too many reagents to be added and the control of the temperature affecting the yield of the reaction is too tedious. Furthermore, corrosive hydrogenchloride gas evolution is quite vigorous for several hours. For the purification of the reaction product, acetic acid coming from the hydrolysis of amide and acetyl chloride should be removed by distillation, which also makes this process laborious.